Phil Mickelson has split from his swing coach Butch Harmon after eight years.
Mickelson won two of his five majors with 72-year-old Harmon as part of his team, but he is winless in the last two seasons.
"I've learned a great deal from him in our eight years together," said the 45-year-old, whose last win was at the 2013 Open.
"It's just that at the moment I need to hear new ideas from a different perspective."
Mickelson also won the 2010 Masters with Harmon as well as 12 of his 42 PGA Tour victories.
But he has dropped down to 25 in the world after recording just five top-10 finishes since his 2013 Open success.
"I've thoroughly enjoyed working with Phil and we've had great success together," said Harmon, who also works with Dustin Johnson and Rickie Fowler.
"Helping him win the Open Championship in 2013 was one of the pinnacles of my career.
"I see nothing wrong with him seeking advice from another source. We're great friends and always will be."
Analysis: 'Reinvigorating effect'
BBC Sport's golf correspondent Iain Carter said: "It is easy to understand Mickelson's decision.
"Although he is hitting the ball further off the tee, he has struggled for accuracy and there has been a significant drop in the number of greens he hits in regulation.
"He has tumbled nearly a hundred places in the PGA Tour rankings in this regard.
"It has put stress on other facets of his game and he clearly feels the need for fresh ideas.
"Aged 45, there is every chance that Mickelson's best years are behind him, but a new coach can have a reinvigorating effect even on the most experienced of players."